1. Field of the Invention
The present invention relates to an optical scanning module, device, and method, and an imaging apparatus.
2. Description of the Related Art
Conventional optical scanning devices employ polygon or galvano-mirrors as deflectors deflecting light beams for scanning. In order to realize an image of a higher resolution and to achieve higher-speed printing, it is necessary to rotate the deflector at a higher rate, which, however, entails a problem of bearing durability or a problem of heat generation or noise caused by windage. This sets a limit to high-speed scanning.
On the other hand, recently, researches on an optical deflector using silicon micromachining have made progress, so that such an optical deflector formed by integrally forming a movable mirror and torsion bars supporting the movable mirror of a semiconductor substrate as disclosed in Japanese Patent No. 2722314 or No. 3011144 has been proposed. The optical deflector disclosed in Japanese Patent No. 2722314 or No. 3011144, which oscillates back and forth using resonance, is therefore operable at a high speed. Further, since the torsion bars of single-crystal silicon are degradation-free, the above-described optical deflector is excellent in high-speed scanning.
The above-described optical deflector gains amplitude by using resonance, thus having the advantage of extremely small power consumption and reduced noise compared with the conventional polygon or galvano-mirror. However, in the case of scanning a surface with a deflected light beam, the above-described optical deflector is moved in the other direction at each end of a scanning line so that a scanning rate is 0 at each end of the scanning line and is maximized at the center thereof, thus having a great variation. Therefore, it is difficult to correct an optical scanning device using the above-described optical deflector with a scanning lens so that scanning is performed at a constant rate on a scanned surface as in the conventional optical scanning device using the polygon mirror.
As will be described later, a maximum swing angle θ0 is inversely related to a resonant frequency fd. As a recording rate becomes higher, the maximum swing angle θ0 becomes smaller. In practice, the maximum swing angle θ0 becomes equal to or smaller than 10°.
Since the scanning rate decreases at an accelerating pace as the swing angle of the movable mirror becomes greater, the movable mirror may be used with the range of a variation in the scanning rate being set relatively narrow by limiting a swing angle θs used for image recording with respect to the maximum swing angle θ0. However, this incurs reduction in an image recording region, causing a problem that a ratio of the swing angle θs used for image recording to the maximum swing angle θ0, that is, an effective scanning rate θs/θ0, becomes low.
The galvano-mirror has a movable coil turnably supported in a magnetic field. The movable mirror is turned in opposite directions by using the equilibrium of the rotational torque of an electromagnetic force and a return spring, the electromagnetic force being generated in the movable coil by supplying an electric current thereto. Compared with the polygon mirror, the galvano-mirror is simple in configuration and small in size.
As previously described, with the recent development of micromachining, a galvano-mirror having a movable mirror and torsion bars supporting the movable mirror formed integrally with each other in a Si substrate has been proposed as disclosed in Japanese Patent No. 2722314. Further, a method of oscillating a movable mirror by using an electrostatic attraction is disclosed in Japanese Patent No. 3011144. According to these inventions, high-speed and highly productive deflectors can be obtained.
Such a movable mirror formed on a Si substrate can cause a light beam to perform scanning at a higher rate than the polygon mirror by matching the frequency of attraction or a repulsive force given to the movable mirror with the natural frequency thereof so that the movable mirror oscillates by resonance.
The swing angle θ of the movable mirror is given by:θ=T/K(K=G·I/L)where G is the modulus of elasticity, I is the geometrical moment of inertia, L is the length, K is the spring constant determined by the length L, and T is the torque given by the electrostatic attraction of torsion bars supporting the movable mirror.
Further, the resonant frequency φ of the movable mirror is given by the following expression:φ=(K/J)1/2where J is the moment of inertia.
The speed or the resonance frequency and the swing angle θ of the movable mirror are inversely related. Generally, the swing angle θ is smaller than or equal to approximately 10°, so that it is difficult for the movable mirror to achieve as large a scanning angle as the polygon mirror (approximately 40°) in terms of its physical property. Therefore, in order to secure a recording width as s substitute for the polygon mirror, it is necessary for the movable mirror to have a large optical path length (a distance between a deflection surface and a scanned surface). This enlarges the size of the entire optical scanning device employing the movable mirror, which is minute as a deflector.